This invention relates generally to a system for an electrical power switch and more particularly to an automatic electrical switch for providing power to a load on a continuous, uninterrupted basis from multiple sources.
1. Field of the Invention
Modern industrial uses for electricity have become more sensitive to power quality. For example, applications such as telecommunications, health care, process control and computing require reliable power that cannot tolerate even brief interruptions. Yet, the availability of reliable power from single sources such as the utility grid has diminished significantly. To address these issues, modern power systems with requirements for high reliability have one or more alternate sources. When power from the primary source is interrupted, one of the secondary sources is tapped without appreciable interruption to the load. This invention provides highly reliable, continuous power to the load from one or more sources, independent of voltage, current and frequency.
2. Problem
With the advent of electronics, control of power to loads with manually activated switches has given way to systems that sense power quality and switch automatically to an alternate source. FIG. I depicts the modern power switch architecture. Modern power switching systems generally fall into two basic categories: those referred to as xe2x80x9cAutomatic Transfer Switchesxe2x80x9d (ATS) and those referred to as xe2x80x9cStatic Transfer Switchesxe2x80x9d (STS). The fundamental difference between the ATS and STS is the types of switches that make and break the electrical connection between the source and the load.
The switching element used by the ATS is the contactor. This is typically a solenoid-actuated device with a single pole per phase and one contactor is typically used per source. The switching operation is performed as follows: first the switch attached to the degraded source is opened; second, once the degraded source is electrically removed from the load the contactor to the alternate source is closed. This xe2x80x9cbreak before makexe2x80x9d or xe2x80x9copen transitionxe2x80x9d action (where the degraded source is removed prior to the alternate source being brought on-line) is required for several reasons: to prevent the alternate source from feeding power back to the other loads on the primary source""s system; to prevent the alternate source from being affected by the condition that may have caused the primary source to malfunction; and to avoid having to synchronize the voltage between the two sources.
Due to the mechanical nature of the ATS switching device, the delay between the opening of the first switch and the closing of the second may be as long as a couple of hundred milliseconds. This type of power interruption is unacceptable in many modern applications. Therefore, an alternative in the form of an STS has been developed to help address this problem.
The STS utilizes semiconductors as the switching element. Most modern systems configure Silicon Controlled Rectifiers (SCRs) in an xe2x80x9canti-parallelxe2x80x9d manner. As such, each phase of each source requires two SCRs; one for conducting current in each direction for a total of twelve SCRs in a three phase STS system with two sources. SCRs are chosen because they provide the lowest xe2x80x9con statexe2x80x9d voltage drop at the current levels that such switches are typically encountered in industrial applications.
Switching operation by an STS is performed by removing a gating signal from the SCRs controlling power from the degraded source and then applying a gating signal to the SCRs controlling power from the alternate source. The sequence of these operations can be controlled to minimize discontinuity in power and prevent the two sources from being connected through the switch.
Typically, the STS will switch power sources within 0.25 cycles of the incoming power signal. However, load and source characteristics can significantly affect the timing and quality of the transition. For instance, when current and voltage are not in-phase, the SCRs (which can only be commutated off during a zero crossing of current through them) may take significantly longer. Even though the STS is faster than ATS in switching power sources, like the ATS, the load side of the STS experiences a brief power loss. Other significant disturbances can occur when the voltage sources are not synchronized and the STS effects an instantaneous phase change on load.
The present invention, the Uninterruptible Transfer Switch (UTS), combines the strengths of the ATS and STS and can provide better switching performance, higher reliability, lower power dissipation, smaller physical size and can switch sources without appreciable power interruption to the load.
The system consists of switches coupled to the power sources. An inverter is coupled to the load side of the switches. As with the ATS, there is a brief period during which the switches of all of the sources are not connected to the load. During this period, the UTS includes an inverter which can include an alternate energy storage device to supply power to the load. The inverter or energy storage device can be powered through an auctioneering circuit consisting of rectifiers that connect to all of the sources. Once the power transfer is complete, the inverter or energy storage device is returned to a standby state until further source switching is required.
Numerous benefits are realized from the present invention. For example, during the source transfer operation, the UTS provides an increased power quality to the load. There is no appreciable power interruption when transferring between sources. Because the voltage across the switch transferring in the new source is nearly zero, switch life is considerably improved. Moreover, because the inverter supplies power during the transfer time, and is typically otherwise on standby, the heat removal apparatus as required by a STS is not necessary. If cooling is used it can be turned on for brief periods and then turned off. This results in increased system reliability and lower operating cost.
In one embodiment of the invention that utilizes mechanical contactors to switch power sources, it is not necessary to have xe2x80x9cactivexe2x80x9d or semiconductor components in the current pathxe2x80x94only copper wire in the choke and the contacts of the mechanical switch. This provides increased reliability and capacity to handle to transient current but with the desirable attributes of switching semiconductors. Additionally, in this embodiment, the power loss in the switch and choke is significantly lower than in switching semiconductors like those in the ATS.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.